1. Field of the Invention
The present invention relates in general to a stator for a dynamo-electric machine of, for example, a three phase induction electric motor, and more particularly to the construction of an armature coil which is wound on a stator.
2. Description of the Related Art
As described in Japanese Patent Examined Application No. Hei 7-44797 for example, a conventional armature coil is constructed in such a way that coil segments, each of which is formed into a general U shape, are successively inserted at intervals of three slots from one end side of a stator iron core and open ends of the coil segments each of which protrudes to the other end side of the stator iron core are joined to each other to obtain predetermined wiring construction.
In this conventional armature coil, V-shaped turn parts of the coil segments are arranged in the circumferential direction on one end of the stator iron core to construct one coil end group, while the joining portions of the open ends of the coil segments are arranged in the circumferential direction on the other end of the stator iron core to construct the other coil end group.
Then, in the conventional armature coil, there is an inconvenience that the axial height of the coil end groups is high so that the stator can not be miniaturized.
In order to solve such an inconvenience, there has been proposed an armature coil in which the coil ends are arranged along the end face of the stator iron core to decrease the axial height of the coil end groups.
FIG. 5 is a developed perspective view showing a main part of an armature coil as the improvement measure which is applied to a stator of a conventional three phase induction electric motor, and FIG. 6 is an exploded perspective view of the armature coil shown in FIG. 5.
In FIGS. 5 and 6, an armature coil 1 is constituted of, for example, windings 2, 3 and 4 of first, second and third phases corresponding to a phase U, a phase V and a phase W, respectively. With respect to each of the winding 2 of the first phase, the winding 3 of the second phase, and the winding 4 of the third phase, a group of large number of copper wires each of which is coated with enamel and which are tied up in a bundle are subjected to the press forming to be formed into a desired shape.
The winding 2 of the first phase is constructed into a crank shape which includes slot accommodating parts 2a which are arranged at predetermined pitches (3P) and bridging parts 2b as the coil ends through each of which the end portions of the adjacent accommodating parts 2a are coupled to each other. Then, each of the slot accommodating parts 2a is formed into the shape having an outer diameter which is roughly equal to an inner diameter of the slot (not shown) of the stator iron core. In addition, each of the bridging parts 2b is formed in such a way that its width (w) in the direction of the slot depth is roughly half the width (2w) in the direction of the slot depth of the slot accommodating part 2a. Thus, the end portions, on one side in the direction of the slot depth, of the adjacent slot accommodating parts 2a are coupled to each other through the associated one of the bridging parts 2b. 
The winding 3 of the second phase is constructed into a crank shape which includes slot accommodating parts 3a which are arranged at predetermined pitches (3P) and bridging parts 3b as the coil ends through each of which the end portions of the adjacent accommodating parts 3a are coupled to each other. Then, each of the slot accommodating parts 3a is formed into the shape having an outer diameter which is roughly equal to an inner diameter of the slot (not shown) of the stator iron core. In addition, each of the bridging parts 3b is formed in such a way that its width (w) in the direction of the slot depth is roughly half the width (2w) in the direction of the slot depth of the slot accommodating part 3a. Thus, the end portions, on the other side in the direction of the slot depth, of the adjacent slot accommodating parts 3a are coupled to each other through the associated one of the bridging parts 3b. 
The winding 4 of the third phase is constructed into a crank shape which includes slot accommodating parts 4a which are arranged at predetermined pitches (3P) and bridging parts 4b as the coil ends through each of which the end parts of the adjacent accommodating parts 4a are coupled to each other. Then, each of the slot accommodating parts 4a is formed into the shape having an outer diameter which is roughly equal to an inner diameter of the slot (not shown) of the stator iron core. In addition, each of the bridging parts 4b is formed in such a way that its width (w) in the direction of the slot depth is roughly half the width (2w) in the direction of the slot depth of the slot accommodating part 4a. Thus, the end parts, on one side in the direction of the slot depth, of the adjacent slot accommodating parts 4a are coupled to the end parts thereof on the other side in the direction of the slot depth through the associated one of the bridging parts 4b. That is, each of the bridging parts 4b is shifted at the intermediate part thereof from one side to the other side in the direction of the slot depth.
The winding 2 of the first phase, the winding 3 of the second phase and the winding 4 of the third phase which are constructed in such a manner as described above are arranged with the slot accommodating parts 2a, 3a and 4a made shifted from each other by one slot (P) to construct the armature coil shown in FIG. 5.
The armature coil 1 which has been constructed in such a manner is formed into a ring-like shape by the bending. Then, divided stator iron cores 5a and 5b are applied from the outer periphery side to the ring-like shaped armature coil 1 in such a way that the slot accommodating parts 2a, 3a and 4a are respectively accommodated into the associated ones of the slots 6, and then the end faces of the divided stator iron cores 5a and 5b are brought into contact with each other to be integrated with each other by the welding, whereby a stator 8 which is shown in FIG. 7 is obtained.
In the stator 8 which has been constructed in such a manner, the winding 2 of the first phase, the winding 3 of the second phase and the winding 4 of the third phase which constitute the armature coil 1 are accommodated at intervals of three slots with the slot accommodating parts 2a, 3a and 4a made shifted from each other by one slot to be wound on the stator iron core 5. Then, the bridging parts 2b of the winding 2 of the first phase are arranged, on the outer periphery side in the radial direction, on the end face of the stator iron core 5, the bridging parts 3b of the winding 3 of the second phase are arranged, on the inner periphery side in the radial direction, on the end face of the stator iron core 5, and the bridging parts 4b of the winding 4 of the third phase are arranged on the end face of the stator iron core 5 in such a way as to be shifted from the inner periphery side in the radial direction to the outer periphery side in the radial direction, thereby constituting the coil end group.
Then, since each of the winding 2 of the first phase, the winding 3 of the second phase and the winding 4 of the third phase is formed into the crank-like shape, the bridging parts 2b, 3b and 4b are arranged along the end face of the stator iron core 5 so that the axial height of the coil end group can be made low.
Since the conventional armature coil employs the coil segments each being formed into the U shape, there is a problem that the axial height of the coil end group is high and hence the stator can not be miniaturized.
In addition, in the armature coil 1 as the improvement measure, the winding 2 of the first phase, the winding 3 of the second phase and the winding 4 of the third phase which constitute the armature coil 1 are wound on the stator iron core 5 in such a way that the bridging parts 2b, 3b and 4b are arranged in the circumferential direction in the form of two rows arranged radially along the end face of the stator iron core 5. Then, the bridging parts 2b of the winding 2 of the first phase are arranged on the end face of the stator iron core 5 in such a way as to be located on the outer periphery side in the radial direction, the bridging parts 3b of the winding 3 of the second phase are arranged on the end face of the stator iron core 5 in such a way as to be located on the inner periphery side in the radial direction, and the bridging parts 4b of the winding 4 of the third phase are arranged on the end face of the stator iron core 5 in such a way as to be shifted from the inner periphery side to the outer periphery side in the radial direction. Then, there arises a problem that the lengths of the winding 2 of the first phase, the winding 3 of the second phase and the winding 4 of the third phase are different from one another, i.e., the resistance values of the windings of the three phases are unbalanced and hence the currents which are caused to flow through the windings of the three phases, respectively, become unbalanced. This unbalance in the currents which are caused to flow through the windings of the three phases, respectively, leads to undesirable torque fluctuation, noise and vibration.
In the light of the foregoing, the present invention has been made in order to solve the above-mentioned problems associated with the prior art and it is therefore an object of the present invention to provide a stator for a dynamo-electric machine in which an axial height of a coil end group is reduced to make miniaturization possible and the unbalance of currents which are caused to flow through the windings of the individual phases, respectively, can be suppressed.
In order to achieve the above object, according to one aspect of the present invention, there is provided a stator of a dynamo-electric machine including: a cylindrical stator iron core formed with a number of slots opening to the inner periphery side at a predetermined pitch in a circumferential direction; and an armature coil having windings of first, second and third phases which are wound in a wave winding manner into every third slot by offsetting the slots, into which the respective windings of the first, second and third phases are inserted, one slot, wherein the windings of the first, second and third phases constitute coil ends in which they protrude from first slot to an end face of the stator iron core outside the slots, extend along the end face of the stator iron core in the circumferential direction while maintaining the radial position, and then enter into second slot three slots away from the first slot, and wherein the coil ends are arranged in the circumferential direction in the form of two layers arranged radially on the end faces of the stator iron core to constitute coil end groups.